Development of eddy-current tangential transducers for inspection of threads of pump-compressor and drill pipes

摘要

Threaded joints of pump-compressor and drill pipes experience a large static and dynamic load during operating process close to elastic limit of materials in the first three thread turns. It may lead to an emergence of cracks in thread groove. Standards regulate an application of magnetic and acoustic methods of thread inspection during repair of pipes requiring consumable materials and good surface preparation. However, it is interesting to apply eddy-current phase inspection method. Traditional circular shape of eddy-current transducer windings that have axis perpendicular to the surface of controlled object does not enable to ensure the necessary sensitivity. The paper discusses questions of use of three-winding transducer with excitation and measuring winding that copy the shape of thread and are set tangentially in the inspection zone. Linear eddy current is excited in the thread groove, the eddy current is perpendicular to the direction of crack in groove. A change of signal phase occurs at the output of differential connected measuring and compensating windings in the area of crack. The paper presents results of modelling, influence of inspected and obstructive parameters and describes the technical characteristics of developed transducers and calibration specimens, inspection methods, processing of results and experience with their application. Threaded joint is the most loaded element in a drill string. Design features of tool joint thread take into account large static and dynamic loads, which arise during its operating process. In addition, profile of a thread with variable height creates natural concentrations of mechanical stresses and spots of fatigue cracks initiations. Stresses arising during operating process receive the maximum value in the last three turns of thread near the runout. Figure 1 shows calculated diagram of stresses at the axis of threaded product. It can be seen that stresses in the thread root of the first three turns of pipe thread from side of runout can approach elastic limit of pipe material.